The large intestine of humans harbors a complex, cell rich, and diverse microbial community consisting of hundreds of different bacterial species. Included within this microbiota are organisms whose presence is associated with, or that contribute to the health of the host, referred to as probiotics. Probiotics are defined as microbial cell preparations or components of microbial cells that have a beneficial effect on the health and well-being of the host. Probiotics have been identified from a number of different genera including, but not limited to, Lactobacilli, Streptococcus and Bifidobacterium, which have many species that are indigenous to the human digestive tract.
Probiotics are thought to exert their beneficial effects by displacing pathogenic enteric bacteria from the intestinal mucosa due, at least, in part, to competitive binding. For example, enteric pathogens such as enteropathogenic Escherichia coli (EPEC), enterotoxigenic E. coli (ETEC), Salmonella enteriditis, Yersina pseudotuberculosis and Listeria monocytogenes must be able to successively colonize an animal's gastrointestinal tract in order to cause disease.